The Structure of a C++ Program

  Introduction

You should have created your first program - Hello World! - by now. Although this is a fairly simple program it has many different parts to it. You need to understand how all the different parts work together.

  Another Look at Hello World!

Let's examine the code for Hello World! in detail.

Here is the code again.

1 #include <iostream.h> 2 int main() 3 { 4 cout << "Hello World!\n"; 5 return 0; 6 }

You should note I have added line numbers.

Line 1 - #include <iostream.h>

The #include directive is an instruction to the preprocessor to include the file iostream.h in the Hello World! program. This file is necessary so that the function cout on line 4 can be used. You can find out more about directives here.

Line 2 - int main()

The program actually start with the function called main(). Every C++ program must start with this function. When your program first starts, main() is called automatically and the code inside it is executed.

Also, after all the code inside any function has finished executing, the function can return a value. In fact, every function is expected to declare what type of value it is going to return In this case, the main() function is declared with the word int before it. This specifies that main() will return an integer value.

This will be discussed further later in the course.

Line 3 - {

All functions and sub routines in C++ keep code enclosed between a pair of curly braces {}. The end curly brace for the function main() is on line 6. In effect, the braces define where the function starts and ends. If you forget to type in a curly brace or type it in the wrong place, you will get a compiler error.

Line 4 - cout << "Hello World!\n";

This is the part of the program that writes the words Hello World! to the command console.

cout is the function that writes output to a screen

<< is the output redirection operator. Whatever follows the output redirection operator is written to the screen. If you want a string of characters written, they must be enclosed in double quotes (").

"Hello World!\n" is the set of characters you want written to the screen. The \n part tells cout to put a new line after the words Hello World!

; most lines of code in C++ end with a semi-colon.

Line 5 - return 0;

As mentioned previously, the function main is expected to return an integer value. Quite often, by convention a programmer will return a 1 to indicate any errors, and a 0 to indicate all is well.

  Extension Work

Carry out the following exercise on your Hello World! program:-

1. Change the line:-

int main()

to

void main()

Do you get a compiler error? Which line of code could you remove to fix the problem?

2. Examine the code below. There is an error in the code, can you spot it?

1 #include <iostream.h> 2 int main() 3 { 4 cout << "There is an error in this program!\n; 5 return 0; 6 }